1. Field of the Invention
The present invention relates generally to an electric energy measuring method and, more particularly, to a method of measuring electric energy by the use of a digital multiplication type electronic watt-hour meter.
2. Description of the Prior Art
FIG. 3 is a block diagram of a digital multiplication type electronic watt-hour meter 20 used for the conventional measurement of electric energy.
In FIG. 3, reference numerals 1 and 2 denote input terminals of an electronic watt-hour meter to which a voltage and a current of a circuit to be measured are inputted respectively, wherein the input terminal 1 is connected to one contact 3a of a first changeover switch 3 while the input terminal 2 is connected to another contact 3b thereof. Meanwhile a common contact 3c of the first changeover switch 3 is connected to one contact 4a of a second changeover switch 4 another contact 4b of switch 4 is connected to a reference potential. And a common contact 4c of the second changeover switch 4 is connected to an input of an A/D converter 5, whose output is connected to a bus line 10 which extends to interconnect the A/D converter 5, a central processing unit (hereinafter referred to as CPU) 6, a read-only memory (ROM) 7 for storing instructions and so forth to be executed by the CPU 6 to operate the electronic watt-hour meter 20, a random access memory (RAM) 8 for storing temporary parameter data, and an interface (I/O) 9.
The I/O is connected via an output line 11 to the first changeover switch 3, the second changeover switch 4 and a control input terminal of the A/D converter 5 while being connected via a signal line 12 to an output terminal 13 of the electronic watt-hour meter 20. The RAM 8 incorporates a register to store a momentary power value computed by the CPU 6.
The operation of the digital multiplication type electronic watt-hour meter having the above structure will now be described below with reference to the flow chart of FIG. 4.
The CPU 6 periodically reads out the sequential instructions stored in the ROM 7 and then decodes and executes such instructions. First, a changeover command signal is outputted via the output line 11 so as to connect the common contact 4c of the second changeover switch 4 to the contact 4b, thereby introducing the reference potential to the A/D converter 5 (step 211). An output digital value of the A/D converter 5 corresponding to the aforesaid potential, i.e. a digital value corresponding to the offset in the A/D converter 5, is fed to the RAM 8 via the bus line 10 and is stored therein (step 213). Upon storage of the digital value in the RAM 8 in step 213, changeover command signals are outputted via the I/O 9 and the output line 11 so as to connect the second changeover switch 4 to the contact 4a and also to connect the common contact 3c of the first changeover switch 3 to the contact 3a respectively, whereby a load voltage applied to the input terminal 1 is introduced to the A/D converter 5 (step 215). The load voltage thus inputted to the A/D converter 5 in step 215 is converted to a digital value corresponding to the load voltage (step 217). Since this digital value includes the offset generated in the A/D converter 5 and so forth, the harmful influence of such offset is removed by subtracting therefrom the aforesaid digital value corresponding to the offset of the A/D converter 5 and stored in step 213, thereby obtaining a digital value corresponding to the input voltage (step 219).
After the offset-removed input voltage is thus calculated in step 219, a changeover command signal is outputted via the I/O 9 and the output line 11 so as to change the connection of the first changeover switch 3 from the contact 3a to the contact 3b, whereby the load current fed to the input terminal 2 is introduced to the A/D converter (step 221). The load current inputted to the A/D converter 5 in step 221 is converted to a digital value corresponding to the load current (step 223). Since this digital value includes the offset generated in the A/D converter 5 and so forth, the aforesaid digital value corresponding to such offset of the A/D converter 5 and stored in step 213 is subtracted to obtain a digital value corresponding to the input current where the harmful influence of the offset has been removed (step 225). After calculation of the digital value corresponding to the offset-free input current in step 225, this digital value is multiplied (step 227) by the digital value obtained previously in step 219, and the result is inputted as a momentary power value of the measured circuit to the register incorporated in the RAM 8 and then is accumulated in the register (step 229). Thereafter the process from step 211 through step 229 is repeated periodically so that the momentary power values free from the offset are inputted to and accumulated in the register, and upon attainment of the accumulation to a predetermined value (step 231), the aforesaid data is fed to the output terminal 13 via the I/O 9 and the signal line 12 (step 233) while the register is cleared (step 235).
In the known electric energy measurement performed by the above-described conventional digital multiplication type electronic watt-hour meter 20, it is desired that the period of serial operations from A/D conversion to multiplication and accumulation be as short as possible in view of enhancing the characteristics relative to distortion in the input waveform. In an exemplary case of repeating 200 times the process from step 211 through step 229 at a frequency of 50 Hz, the period needs to be 100 .mu.sec.
However, for repeating 200 times the steps 211 through 229 within a period of 100 .mu.sec, the CPU 6 and other components employed should be capable of performing sufficiently fast operations. Consequently, a problem has been existent heretofore that high-precision measurement is impossible by the use of an inexpensive low-loss digital multiplication type electronic watt-hour meter.
The present invention has been accomplished in an attempt to eliminate the problem mentioned above.